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Oxygen Minimum Zones (Omzs) in the Modern Ocean

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Oxygen Minimum Zones (Omzs) in the Modern Ocean Progress in Oceanography 80 (2009) 113–128 Contents lists available at ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean Oxygen minimum zones (OMZs) in the modern ocean A. Paulmier a,b,*, D. Ruiz-Pino b a LEGOS/CNRS 18, Av. Ed. Belin, 31401 Toulouse Cedex 9, France b LOCEAN, Université P&M Curie, Courrier 134, 4 pl. Jussieu, 75252 Paris Cedex 05, France article info abstract Article history: In the modern ocean, oxygen minimum zones (OMZs) are potential traces of a primitive ocean in Received 6 September 2007 which Archean bacteria lived and reduced chemical anomalies occurred. But OMZs are also keys to Received in revised form 1 August 2008 understanding the present unbalanced nitrogen cycle and the oceans’ role on atmospheric greenhouse Accepted 4 August 2008 control. OMZs are the main areas of nitrogen loss (as N ,NO) to the atmosphere through denitrifi- Available online 17 August 2008 2 2 cation and anammox, and could even indirectly mitigate the oceanic biological sequestration of CO2. It was recently hypothesized that OMZs are going to spread in the coming decades as a conse- Keywords: quence of global climate change. Despite an important OMZ role for the origin of marine life and for Oxygen minimum zones (OMZs) the biogeochemical cycles of carbon and nitrogen, there are some key questions on the structure of Oxygen Global ocean OMZs at a global scale. There is no agreement concerning the threshold in oxygen that defines an Denitrification OMZ, and the extent of an OMZ is often evaluated by denitrification criteria which, at the same time, Biogeochemistry are O2-dependent. Our work deals with the identification of each OMZ, the evaluation of its extent, volume and ver- tical structure, the determination of its seasonality or permanence and the comparison between OMZs and denitrification zones at a global scale. The co-existence in the OMZ of oxic (in its boundaries) and suboxic (even anoxic, in its core) conditions involves rather complex biogeochemical processes such as strong remineralization of the organic matter, removal of nitrate and release of nitrite. The quan- titative OMZ analysis is focused on taking into account the whole water volume under the influence of an OMZ and adapted to the study of the specific low oxygen biogeochemical processes. A characterization of the entire structure for the main and most intense OMZs (O2 <20lM reaching 1 lM in the core) is proposed based on a previously published CRIO criterion from the eastern South Pacific OMZ and including a large range of O2 concentrations. Using the updated global WOA2005 O2 climatology, the four known tropical OMZs in the open ocean have been described: the Eastern South Pacific and Eastern Tropical North Pacific, in the Pacific Ocean; the Arabian Sea and Bay of Bengal, in the Indian Ocean. Moreover, the Eastern Sub-Tropical North Pacific (25–52°N) has been identified as a lesser known permanent deep OMZ. Two additional seasonal OMZs at high latitude have also been identified: the West Bering Sea and the Gulf of Alaska. The total surface of the permanent OMZs is 30.4 millions of km2 ( 8% of the total oceanic area), and the volume of the OMZ cores (10.3 millions of km3) corresponds to a value 7 times higher than previous evaluations. The volume of the OMZ cores is about three times larger than that of the associated denitrification zone, here defined as NMZ (‘nitrate deficit or NDEF > 10 lM’ maximum zone). The larger OMZ, relative to the extent of deni- trification zone, suggests that the unbalanced nitrogen cycle on the global scale could be more intense than previously recognized and that evaluation of the OMZ from denitrification could underestimate their extent. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction (1972). OMZs correspond to subsurface oceanic zones (e.g., at 50– 100 m depth in the Arabian Sea; Morrison et al., 1999) and reaching The interest in oxygen minimum zones (OMZs), characterized ultra-low values of O2 concentration (e.g. <1 lM; Karstensen et al., herein as O2-deficient layers in the ocean water column, is quite re- 2008). OMZs, because of their intensity and shallowness, are, a priori, cent, since the appearance of the name ‘‘OMZ” in Cline and Richards different from the relatively well known ‘‘classical O2 minimum”, which is 50 times more oxygenated than OMZs and found at intermediate depths (1000–1500 m) in all the oceans (Wyrtki, * Corresponding author. Address: LEGOS/CNRS 18, Av. Ed. Belin, 31401 Toulouse Cedex 9, France. Tel.: +33 (0)561333007; fax: +33 (0)561253205. 1962). Note that in the present study, an OMZ is defined as being E-mail address: [email protected] (D. Ruiz-Pino). ‘‘more intense’’, when the O2 concentrations in its core are lower. 0079-6611/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2008.08.001 114 A. Paulmier, D. Ruiz-Pino / Progress in Oceanography 80 (2009) 113–128 1.1. OMZ specificities for marine biogeochemistry and ecosystems conditions, OMZs would increase or intensify, according to obser- vations in recent decades (e.g., Stramma et al., 2008). But evalua- OMZs have been mainly known for playing an essential role in tions or predictions of OMZs variation over paleoclimatic periods, the global nitrogen cycle, in which various chemical species, since the anthropocene era or in the future, cannot be validated according to their degree of oxidation (e.g. ammonium, NH4þ; without a reference state, and the report of all the existing OMZs nitrite, NO2À; nitrate, NO3À; nitrous oxide, N2O; dinitrogen, N2), detected in the modern ocean taking into account improvements and different bacterial processes intervene. Under oxic conditions, in O2-measurement techniques. but also at the upper boundary (oxycline) of an OMZ, nitrification Despite the important role of OMZs in understanding primitive transforms NH4þ into NO3À. But OMZs are especially associated with marine life and chemistry, as well as in the carbon (C) and nitrogen denitrification, which is a bacterial process occurring only in (N) cycles, little knowledge has been obtained on the extent and O2-deficient regions (e.g., Codispoti et al., 2001). Denitrification vertical structure of these oceanic ‘‘curiosities”. This is mainly converts NO3À, one of the main limiting nutrients in the ocean, into due to the following difficulties: (i) few available O2 data obtained gaseous nitrogen (N as, for example, N2O, N2) which is lost to the with a low enough detection limit (<1 lM) and accuracy (<2 lM), atmosphere and contributes to the oceanic nitrate deficit (N/ owing to the present limitations in the sampling and analysis tech- P 14.7; e.g., Tyrrell, 1999). However, recently, an unknown pro- niques linked to the low O concentration; (ii) the choice of a 2 cess in the ocean has been observed, first in sediments and then unique criterion for all OMZs, since the nature of this criterion in the water column in the OMZs (e.g., Kuypers et al., 2003): the often depends on research interest (e.g., specific low-O2 biogeo- anaerobic oxidation of NH4þ using NO2À (anammox); this imposes chemistry process studies have to take into account an O2 concen- a complete revision of the global nitrogen cycle (e.g., Arrigo, tration lower than 20 lM, but the influence of physical processes 2005). OMZs are also involved in the cycle of very important cli- do not make it necessary to include suboxic and anoxic condi- matic gases: (i) production of 50% of the oceanic N O (e.g., Bange tions); (iii) the criteria could be different for each OMZ region: 2 et al., 1996); (ii) production of H2 S (e.g., Dugdale et al., 1977) and for example, the OMZs in the Northeastern Atlantic ocean is ex- CH4 (e.g., Cicerone and Oremland, 1988), episodically or for OMZs cluded when a threshold of 20 lM is used (Helly and Levin, in contact with sediments; (iii) limitation of atmospheric CO2 2004). Different terms and thresholds have been used to described sequestration by the ocean: directly as an end-product of reminer- the overall low O2 conditions. Suboxia has been mainly defined by alization (Paulmier et al., 2006) or indirectly through limitation of biologists and biogeochemists as a transition layer from O2-to total primary production due to the N loss (see hypothesis of Fal- NOÀ-respiration, with thresholds between 0.7 lM (e.g., Yakusev 3 kowski, 1997); (iv) potential DMS consumption due to higher bac- and Neretin, 1997) and 20 lM (e.g., Helly and Levin, 2004). Hypox- terial activity (Kiene and Bates, 1990). Chemically, OMZs are ia implies O2 conditions under which macro-organisms cannot live: associated with acidification (low pH 7.5 SWS; Paulmier, 2005), 8 lM for Kamykowski and Zentara (1990), but up to 40 lM and reduced conditions (Richards, 1965) favoring reduced chemi- depending on the species considered, such as anchovy (e.g., Gray cal species (e.g., Fe(II) or Cu(I) potentially stimulating photosynthe- et al., 2002). Dysoxia (O2 <4lM) and microxia (O2 <1lM; Levin, sis or N2O production). 2002) are associated with a sharp O2 transition for the large organ- OMZs have also increased interest in biological and ecosystem isms, such as fishes. Anoxia (O2 < 0.1 lM; Oguz et al., 2000) is defined studies. Because of similarities between Archean bacteria and those by transition from NO3-respiration to sulphate-reduction. living in the OMZs (Zumft, 1997), OMZs could be considered as ana- The first global study providing information on where water logues of the primitive anoxic ocean in which life is widely thought column OMZs can be located is that of Kamykowski and Zentara to have first appeared.
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